Geological faults occur when there has been a fracture along which the blocks of the earth's crust (e.g., fault blocks) on either side have moved relative to one another parallel to the fracture (e.g., the fault plane). By definition, the fault block that is above the fault plane is considered the hanging wall and the fault block that is below the fault plane is defined as the footwall. Different types of faults are classified based on the orientation of the fault blocks. For example, a “normal fault” occurs when the hanging wall moves down relative to the footwall and may occur when there is an expansion of the crust. Alternatively, a “reverse fault” occurs when the hanging wall moves up relative to the footwall and occurs when the crust is compressed. Complex fault topologies may also have cross faults in which two faults cross each other.